In recent years, not only industrial robots but also consumer robots for taking various roles are researched and developed lively. Among the robots, humanoid robots which can walk upright are expected to be able to replace human action. Such a humanoid robot has joints in leg portions, arm portions, a neck portion, and the like and can make movements similar to human movements.
As joints for the robot, there are various joints having one to three or higher degrees of freedom. The degree of freedom refers to the number of directions in which the joint can move. For example, a joint which can rotate only about a pitch axis has one degree of freedom and a joint which can rotate about two axes, i.e., a pitch axis and a yaw axis has two degrees of freedom. In a prior-art articular structure for a robot, it is common practice to combine two or more rotary servomotors in order to achieve a joint with two or higher degrees of freedom. In other words, two or more rotary servomotors are prepared and combined so that rotary shafts of the rotary servomotors are perpendicular to each other (e.g., refer to Patent Literature